Entirely or partially electrically driven motor vehicles or electric vehicles or hybrid vehicles which combine electric vehicle drives and internal combustion engines have reduced emissions compared to conventional motor vehicles with an internal combustion engine. In this context one or more electric machines can, on the one hand, be fed from a battery and used as an engine or drive, and, on the other hand, use as a generator or brake is also possible. This makes it possible to convert at least a portion of the kinetic energy of the vehicle during a braking operation into electrical energy and to store it in a battery.
The drag torque of the electric machine or machines which is/are operated as a generator and therefore the deceleration which can be achieved may be limited here for various reasons: the rotational speed dependency, described in a characteristic diagram, of the power of an electric machine, such as, in particular, the power drop at low rotational speeds, leads to a situation in which, for example at speeds below 10 km/h, it is no longer possible to build up any generator braking torque. The possible charge current of the battery can also limit the power which can be achieved. In particular, in the case of a relatively long journey through a pass it is possible that the battery is fully charged and regenerative braking is no longer possible. In addition, thermal overloading of the power electronics and/or battery has to be avoided.
As generally in the case of braking processes, a low coefficient of friction of the underlying surface makes brake slip control necessary. Depending on the configuration of the electric drives and, in particular, in the case of an electric drive which acts only on the rear axle, it may be necessary to limit the generator braking torque in order to ensure driving stability. Suitable braking force distribution is particularly required in the case of braking operations with high deceleration. The transmission ratio can also limit the achievable deceleration if the electric drive is connected to the wheels via a transmission.
In order to ensure sufficient deceleration in any situation, the vehicle wheels must therefore be additionally equipped with conventional friction brakes. For this reason, hybrid vehicles and electric vehicles generally use a regenerative brake system which splits the total braking torque during a normal braking process (without the intervention of driving stability controllers) into two portions: a generator braking torque, which is generated by the electric vehicle drive or drives and acts on the wheels via the drive train, and a friction braking torque, in the case of which the kinetic energy of the vehicle is converted into thermal energy as a result of friction of the brake linings at the brake disc. This portion of the kinetic energy which is converted into heat is therefore “lost” via friction during the braking process and limits the efficiency of the recuperation, for which reason a maximum portion of generator braking torque is aimed at in order to maximize the energy recover during a braking process and therefore reduce the consumption of primary energy by the vehicle. An exemplary regenerative brake system and a corresponding actuation method are described in DE 102011003346 A1, which is incorporated by reference.
Friction brake systems have a large number of electronic control devices which increase the driving safety by means of driving stability controllers. An exemplary system for controlling driving stability is known from EP 0792228 B1, which is incorporated by reference.
If a brake slip controller (also known as an ABS, i.e. anti-lock brake system) has to intervene during a full braking operation or a braking to a low coefficient of friction, in order to avoid locking of a wheel, said brake slip controller generally acts only on the portion of the braking torque which is generated by friction brakes. This is due to the fact, inter alia, that the change in a braking torque which is requested via a vehicle data bus (for example CAN bus) is implemented with a time constant which exceeds a typical slip control cycle. As a result, during an ABS control process a generator braking torque is usually not produced with respect to the driving stability or the maintenance of a suitable braking force distribution.
WO 2011/015422 A1, which is incorporated by reference, discloses a method for controlling a wheel brake slip for a vehicle having an electric drive, in which a brake signal is detected by means of a slip control device which generates an electric drive signal and a friction brake signal in order to control a predetermined slip value. The electric drive signal is transmitted to an electric drive control device which correspondingly activates the electric drive in order to generate an electric drive torque, and the friction brake signal is transmitted to a friction brake control device which activates a friction brake of the wheel in accordance with the friction brake signal. Highly dynamic control of the electric drive torque is preferably carried out at least partially in the electric drive control device, wherein, in particular, an electric drive rotational speed is detected and evaluated. Since the slip control takes place at least partially in the control device of the electric drive, the implementation must be specially adapted to the current vehicle.